So, I find Harbeth to be slightly warm yet very natural/transparent. However I do not find it "Fast". I had Totem and KEF speakers and listened to many B&W's and found them to sound 'Fast" - but can I describe 'Fast" for you? Perhaps Lively is closest. I would suggest listening to as many speakers as you can and you'll eventually find adjectives that work for you.
For instance, I didn't understand a 'grainy' sound said about DAC's until I had heard enough of them and suddenly I heard one that sounded 'grainy".